Photometric methods for detecting components in a sample, such as a sample of blood, by exposing the sample to radiation and by measuring the absorbance of the radiation by the sample are well known.
For example, such methods are widely used within the field of diagnostic analysis to determine concentrations of body constituents based on optical measurements. The Lambert-Beer's law applies to optical measurements where the transmission of radiation from a radiation source through a sample is determined by a detector, i.e. the absorbance is proportional to the concentration of absorbing species in the sample as well as to the sample thickness, i.e. the path length of the radiation path through the sample. The proportionality factor is referred to as the extinction coefficient.
Application of the Lambert-Beer's law to determine constituents of unknown concentration thus requires knowledge of the path length (along or in combination with the extinction coefficient).
In a measuring apparatus the path length is often defined by the dimensions of a sample chamber. When such chambers have a long lifetime and may be used for a large number of measurements, the dimensions may be determined, e.g. during a calibration process using a sample having a constituent of a known concentration. The same applies to sample chambers that may be manufactured with a high degree of precision. However, when the sample chambers have a shorter life time or when their dimensions are not stable over time or vary from chamber to chamber, frequent calibrations are required, thus resulting in a decreased efficiency of the measuring system. Nevertheless, it may be desirable to use sample chambers having a shorter lifetime, single-use sample chambers, or sample chambers being manufactured with larger tolerances, as these may be considerably less expensive to manufacture.
U.S. Pat. No. 6,442,411 discloses a method of in vivo analysis of blood constituents like hemoglobin and glucose. This prior art method uses the water content in blood samples as an internal reference for the purpose of determining the optical path length by means of a differential determination of water during the systolic and diastolic portions of the arterial pulsation. According to U.S. Pat. No. 6,442,411, the variability of the water concentration in blood is 1.8% around the average level. Whereas such level of precision may suffice in some applications, it would generally be desirable to further increase the precision of the measurements of concentrations of constituents of a sample. For example, for many in vitro analysis purposes, a higher precision is often desired.